This invention relates to a tube coupling and, more particularly, to a face seal tube coupling used in an ultraclean environment.
There are many different fittings available which can join together glands or conduits. One fitting that has proven to be successful in ultraclean environments or very clean rooms ("VCR's") employs a metal gasket in combination with annular sealing beads which project from the ends of tubular members. U.S. Pat. No. 4,854,557 to Leigh and U.S. Pat. No. 4,838,583 to Babuder generally disclose the use of gaskets in face seal fittings between tubular members having annular sealing beads which project in an axial direction from the radial sidewalls. Minimizing the incidence of particulate matter is particularly important in industries such as semiconductor manufacturing and a number of different fittings have been designed for these special applications. Fittings required for applications in ultraclean environments or VCR's have been specifically designed to join tubular members in a manner that minimized the introduction of particulate matter. One successful design involved the modification of a fitting using annular beads and is directed at the reduction of the void spaces or crevices formed between the non-sealing surfaces of the gasket and the radial endwalls of the tubular members. These fittings, known as dead zero volume fittings, reduce the size of voids and, thus, the ability of particulate matter flowing through the fitting to become trapped in voids which could later be abruptly released and cause a high concentration of particulate matter to be introduced into the ultraclean environment.
Other fittings which have been designed for VCR applications attempt to control the position of the components of the fittings as they are drawn together so as to reduce the friction between the sealing surfaces of the tubular members and the sealing surface of the face gasket. It is believed that friction and excess pressure between the sealing elements from forces applied to the gasket can cause scratches or other deformation in the sealing surfaces which contributes to the introduction of undesirable particulate matter. Since the optimal level of particulate matter in such environments is zero, any manner in which the introduction of particulate matter is reduced is a desirable development. The improvements in face seal fittings which have been developed for VCR applications still were subject to limitations. When these fittings were refined to reduce the dead zero volume, the sealing area was reduced. As disclosed in the copending application of McGarvey, Ser. No. 07/652,225, the modification of the semicircular sealing beads in a conventional face seal fitting provided an arcuate exterior profile of the sealing bead with a reduced area available to form a seal. A modified gasket with a beveled face was also disclosed which was designed to engage the new sealing bead structure. The new fitting reduced the available surface area which formed the seal and correspondingly the unit force to surface area or pounds per square inch of pressure on the components increased.
In fittings employing the new designs, it was sometimes only possible to disassemble or reassemble the joints a limited number of times before either the gasket or tubular members would deform to such an extent that the components could no longer form a seal. It has been discovered that when the components are assembled, the parts are often tightened to an unnecessary degree. Overtightening problems were exacerbated in those fittings with reduced sealing surface area. In those fittings, it is difficult to determine with precision the point at which the tubular members and gasket form a satisfactory seal and, as a result, an assembler will continue to tighten the components after a satisfactory seal is formed to ensure a seal is achieved.
After a satisfactory seal is achieved, further rotation of the coupling nuts forces the tubular members to continue to be drawn together and into the gasket. This continued pressure on the components further deforms the sealing surfaces to an extent beyond the elastic limits of the metal which the components are comprised. Deformation to this extent is a suspected source of undesirable particulate matter. The deformation also shortens the effective life of the components because the surfaces are needlessly worn and becomes permanently deformed. Worn or deformed parts will not continue to make an effective seal upon repeated assembly. Overtightening of the gasket results in a shorter life of the sealing surface components because the components will permanently deform and wear at a faster rate. Overtightening is also suspected to be a source of particulate matter because extreme deformation of the components may release microscopic metal fragments into the systems. Overtightening of the seal can also put unnecessary stress upon other components of the fitting, such as the coupling bolt and tubular members thus accelerating fatigue of the entire assembly.
In fittings which employ other types of gaskets where overtightening was identified as a problem, simple spacers external to the gasket were employed or it was recommended that the assembler use a torque wrench. In the past, the problems associated with overtightening of face seal fittings were often simply ignored. Others attempted to achieve proper tightness by trial and error or crudely estimating how far to rotate the coupling nut beyond fingertight. In ultraclean environments, these imprecise methods to prevent overtightening proved to be unsatisfactory.
There is thus a need for a reliable and inexpensive solution that will effectively prevent the overtightening of face seal fittings. Improving the useable life of fittings typically used in ultraclean rooms is particularly desirable since many of the tubular elements are permanently welded into the systems thus making replacement a time consuming and expensive process. Effectively preventing overtightening will both extend the useful life of the components and minimize the incidence of particulate matter.